In the parent patent and related patents, derived from the parent applications, (for example U.S. Pat. Nos. 5,160,550, 5,160,794, 5,230,748, and 5,174,830) there is an extensive discussion of the prior art relating to NbTi, NbTiTa and other superconducting systems particularly those involving artificial pinning centers. This background is incorporated in its entirety by reference.
In the manufacture of superconducting wire for very high field magnets, such as those used in the proposed Superconducting Super Collider (SSC), or the proposed Large Hadron Collider (LHC), the superconductor must remain superconducting in very high magnetic fields in the range of 5 to 9 tesla. It is also recognized by those familiar in the art that such material must be produced from large billets, up to 300 mm in diameter, in order to achieve cost effective economies of scale. Further, as has been described in the parent applications the final reacted Nb+Ti layer thickness must be in the range of 100 nm in order to achieve optimal conductor Jc's. Thus for a given final wire design, the ratio of the final wire diameter to the optimal layer thickness (100 nm) must be the same for the ratio of the starting multifilamentary restack billet diameter (300 mm) to the initial layer thickness because the reduction in diameter is the same as the reduction in layer thickness.
For example, the conductor design for the SSC called for a final wire size of 0.648 mm in diameter. The reduction in diameter from 300 mm multifilamentary billet to 0.648 mm wire is a factor of 463 times. Thus, the initial Nb+Ti layer thickness in the multifilament billet must be 46.3 microns because the final layer thickness must be around 100 mn for optimal Jc's (463.times.100 nm). Such a large initial layer thickness poses special problems for fabricability and current density. These problems can be overcome through modifications described in the present invention.
When the layers within a Nb/Ti composite filament are very thick, it is neither practical nor effective to generate much of the requisite degree of diffusion within the filament prior to extrusion in the manner of a preferred embodiment of the invention described in the parent patents. Under the laws of solid state diffusion, the distance of diffusion at a given temperature increases with the square root of time. To diffuse twice the distance within a composite at a given temperature, one would require four times the length of time. So when the niobium and titanium layers of a filament are very thick, it can require prohibitively long periods of time to achieve the required amount of diffusion. Of course, one can increase the temperature to speed diffusion. (By the solid state diffusion laws, increasing the temperature increases the rate of diffusion in an exponential manner.) While in principle this is an effective approach to thick layers, in practice the temperature must be so high that grain growth and grain boundary diffusion occurs, especially within the titanium layers. This reduces the homogeneity and the degree of order within the structure. The consequence is a degradation of J.sub.c performance for the final conductor due to less efficient flux pinning.
As it is not desirable to optimally diffuse thick niobium and titanium layers prior to extrusion, one is left with a processing route wherein much of the diffusion heat treatment is performed after extrusion. Such a scheme entails extrusion of filaments containing essentially undiffused niobium and titanium layers (FIG. 2A). This presents problems because the Nb/Ti structure does not flow during extrusion in the same way as an optimally diffused structure. In fact, it has been found that filaments containing essentially undiffused niobium and titanium are unusually susceptible to shear fracture during extrusion. If not actually fractured, the filaments often display highly distorted layers. In either case, J.sub.c performance is greatly depressed.
For those familiar with the art, the design of the proposed SSC conductor poses significant difficulties in producing the fine filaments (6 microns) due to problems of sausaging and intermetallic formation of the filaments in the copper matrix. The same concerns exist within the present invention, thus the same care must be taken in avoiding these problems. The niobium and titanium system shall be used as the exemplar throughout this discussion. It will be understood, however, that the principles discussed could be applied to many alternate systems as discussed in the parent patents.